//	-toolexec 'cmd args'
//		a program to use to invoke toolchain programs like vet and asm.
//		For example, instead of running asm, the go command will run
//		'cmd args /path/to/asm <arguments for asm>'.
//		The TOOLEXEC_IMPORTPATH environment variable will be set,
//		matching 'go list -f {{.ImportPath}}' for the package being built.
//
// The -asmflags, -gccgoflags, -gcflags, and -ldflags flags accept a