mips32: add support for FPXX mode

mips32: add support for FPXX mode

With this patch, MIPS32 Valgrind compiled with -mfpxx can handle all types
(regarding FP_ABI flag) of MIPS32 ELFs.

- Functions arch_elf_pt_proc() and arch_check_elf() are added to elf reader
  according to linux/fs/binfmt_elf.c from Linux 4.1;
- Processing .MIPS.abiflags section and initializing appropriate FPU mode
  for MIPS32 are added;
- Emulation of prctl(GET/SET_FP_MODE) sys-calls are implemented for MIPS32.

Patch by Aleksandar Rikalo <Aleksandar.Rikalo@imgtec.com>

Related VEX change: r3243.

This implements BZ#366079.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@15934