Fix SYS_FUNC and SEN macros for musl

Fix SYS_FUNC and SEN macros for musl

The syscall_name argument was subject to macro expansion because
it was passed down to other macros before it was prefixed.

musl libc defines lfs64 names as macros (e.g. fstat64 as fstat)
so SYS_FUNC(fstat64) was expanded to sys_fstat.

This change adds the prefix before the name is passed to other macros,
i.e. the argument of SYS_FUNC_NAME is already prefixed with sys_.

* defs.h (SYS_FUNC): Add sys_ prefix to SYS_FUNC_NAME's argument.
(SYS_FUNC_NAME): Do not add sys_ prefix to MPERS_FUNC_NAME's argument.
* linux/ia64/syscallent.h (SYS_FUNC_NAME): Do not add sys_ prefix
to MPERS_FUNC_NAME's argument.
* syscall.c (SEN_NAME): Remove.
(SEN): Replace SEN_NAME call with its definition.  Add sys_ prefix
to SYS_FUNC_NAME's argument.