gallium: Expand libc check to be platform OS check

endianness on musl based systems can also be detected in same way as glibc, therefore check for __linux__ define instead of __GLIBC__

Fixes build on musl/mips

| ../../../../git/src/gallium/include/pipe/p_config.h:171:2: error: #error Unknown Endianness
| #error Unknown Endianness
|  ^~~~~