Add initial RISC-V support

Add initial RISC-V support

Issue #208 (bdwgc).

* include/private/gcconfig.h [__riscv && LINUX && __riscv_xlen==32]
(RISCV32, mach_type_known): Define macro.
* include/private/gcconfig.h [__riscv && LINUX && __riscv_xlen==64]
(RISCV64, mach_type_known): Define macro.
* include/private/gcconfig.h [RISCV32 || RISCV64] (CPP_WORDSZ,
MACH_TYPE, ALIGNMENT): Likewise.
* include/private/gcconfig.h [(RISCV32 || RISCV64) && LINUX] (OS_TYPE,
DATASTART, LINUX_STACKBOTTOM, DYNAMIC_LOADING): Likewise.
* include/private/gcconfig.h [(RISCV32 || RISCV64) && LINUX]
(__data_start): Declare external variable.