re PR target/38598 (MIPS extendsidi2 does not have a LO alternative)

re PR target/38598 (MIPS extendsidi2 does not have a LO alternative)

gcc/
	PR target/38598
	* config/mips/mips.md (extendsidi2): Add an "l" alternative.
	Update commentary.

gcc/testsuite/
	PR target/38598
	* gcc.target/mips/madd-7.c: Remove -mlong32.
	* gcc.target/mips/msub-7.c: Likewise.

From-SVN: r171572