gcc: xtensa: implement bswapsi2, bswapdi2 and helpers

2020-12-20  Max Filippov  <jcmvbkbc@gmail.com>
gcc/
	* config/xtensa/xtensa.md (bswapsi2, bswapdi2): New patterns.

gcc/testsuite/
	* gcc.target/xtensa/bswap.c: New test.

libgcc/
	* config/xtensa/lib1funcs.S (__bswapsi2, __bswapdi2): New
	functions.
	* config/xtensa/t-xtensa (LIB1ASMFUNCS): Add _bswapsi2 and
	_bswapdi2.