LoongArch: add new configure option --with-strict-align-lib

LoongArch: add new configure option --with-strict-align-lib

LoongArch processors may not support memory accesses without natural
alignments.  Building libraries with -mstrict-align may help with
toolchain binary compatiblity and performance on these implementations
(e.g. Loongson 2K1000LA).

No significant performance degredation is observed on current mainstream
LoongArch processors when the option is enabled.

gcc/ChangeLog:

	* config.gcc: use -mstrict-align for building libraries
	if --with-strict-align-lib is given.
	* doc/install.texi: likewise.