bionic/libc: fix getsid() syscall number for mips

According to libc/kernel/arch-mips/asm/unistd.h, the syscall number
for mips is 151, not 147.

Only bionic/libc/SYSCALLS.TXT was manually edited;
bionic/libc/include/sys/linux-syscalls.h was generated from
SYSCALLS.TXT by running bionic/libc/tools/gensyscalls.py.

Change-Id: Ia14da35acbc36aafd44a059146a37271bd358ae2