Allow the target architecture to be set in the environment.

scripts/Mercury.config.in:
scripts/Mercury.config.boostrap.in:
	Add the environment variable MERCURY_TARGET_ARCH, which allows
	the compiler's target architecture to be overridden in the
	environment.

compiler/options.m:
	Add the option --target-arch as a synonym (and eventually, replacement)
	for the --fullarch option.