[BPF] Don't fail for static variables

[BPF] Don't fail for static variables

Currently, the LLVM will print an error like
  Unsupported relocation: try to compile with -O2 or above,
  or check your static variable usage
if user defines more than one static variables in a single
ELF section (e.g., .bss or .data).

There is ongoing effort to support static and global
variables in libbpf and kernel. This patch removed the
assertion so user programs with static variables won't
fail compilation.

The static variable in-section offset is written to
the "imm" field of the corresponding to-be-relocated
bpf instruction. Below is an example to show how the
application (e.g., libbpf) can relate variable to relocations.

  -bash-4.4$ cat g1.c
  static volatile long a = 2;
  static volatile int b = 3;
  int test() { return a + b; }
  -bash-4.4$ clang -target bpf -O2 -c g1.c
  -bash-4.4$ llvm-readelf -r g1.o

  Relocation section '.rel.text' at offset 0x158 contains 2 entries:
      Offset             Info             Type               Symbol's Value  Symbol's Name
  0000000000000000  0000000400000001 R_BPF_64_64            0000000000000000 .data
  0000000000000018  0000000400000001 R_BPF_64_64            0000000000000000 .data
  -bash-4.4$ llvm-readelf -s g1.o

  Symbol table '.symtab' contains 6 entries:
     Num:    Value          Size Type    Bind   Vis      Ndx Name
       0: 0000000000000000     0 NOTYPE  LOCAL  DEFAULT  UND
       1: 0000000000000000     0 FILE    LOCAL  DEFAULT  ABS g1.c
       2: 0000000000000000     8 OBJECT  LOCAL  DEFAULT    4 a
       3: 0000000000000008     4 OBJECT  LOCAL  DEFAULT    4 b
       4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
       5: 0000000000000000    64 FUNC    GLOBAL DEFAULT    2 test
  -bash-4.4$ llvm-objdump -d g1.o

  g1.o:   file format ELF64-BPF

  Disassembly of section .text:
  0000000000000000 test:
       0:       18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00         r1 = 0 ll
       2:       79 11 00 00 00 00 00 00         r1 = *(u64 *)(r1 + 0)
       3:       18 02 00 00 08 00 00 00 00 00 00 00 00 00 00 00         r2 = 8 ll
       5:       61 20 00 00 00 00 00 00         r0 = *(u32 *)(r2 + 0)
       6:       0f 10 00 00 00 00 00 00         r0 += r1
       7:       95 00 00 00 00 00 00 00         exit
  -bash-4.4$

  . from symbol table, static variable "a" is in section #4, offset 0.
  . from symbol table, static variable "b" is in section #4, offset 8.
  . the first relocation is against symbol #4:
    4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
    and in-section offset 0 (see llvm-objdump result)
  . the second relocation is against symbol #4:
    4: 0000000000000000     0 SECTION LOCAL  DEFAULT    4
    and in-section offset 8 (see llvm-objdump result)
  . therefore, the first relocation is for variable "a", and
    the second relocation is for variable "b".

Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354954 91177308-0d34-0410-b5e6-96231b3b80d8