move O_PATH definition back to arch bits

while it's the same for all presently supported archs, it differs at
least on sparc, and conceptually it's no less arch-specific than the
other O_* macros. O_SEARCH and O_EXEC are still defined in terms of
O_PATH in the main fcntl.h.

add O_TMPFILE flag, new in linux 3.11

definition in linux:
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
where __O_TMPFILE and O_DIRECTORY are arch specific

add missing F_GETOWNER_UIDS flag to bits/fcntl.h

fcntl.h: O_SEARCH was missing for powerpc

put some macros that do not differ between architectures in the
main header and remove from bits.
restructure mips header so it has the same structure as the others.

fix numerous mips abi constant definition mismatches

add O_EXEC open mode

the linux O_PATH mode provides the necessary semantics for both the
O_SEARCH and O_EXEC modes defined and required by POSIX 2008.

add O_PATH/O_SEARCH support to fcntl.h

I'm not 100% sure that Linux's O_PATH meets the POSIX requirements for
O_SEARCH, but it seems very close if not perfect. and old kernels
ignore it, so O_SEARCH will still work as desired as long as the
caller has read permissions to the directory.

initial version of mips (o32) port, based on work by Richard Pennington (rdp)

basically, this version of the code was obtained by starting with
rdp's work from his ellcc source tree, adapting it to musl's build
system and coding style, auditing the bits headers for discrepencies
with kernel definitions or glibc/LSB ABI or large file issues, fixing
up incompatibility with the old binutils from aboriginal linux, and
adding some new special cases to deal with the oddities of sigaction
and pipe syscall interfaces on mips.

at present, minimal test programs work, but some interfaces are broken
or missing. threaded programs probably will not link.