cmd/compile: resolve TODO of Mpflt.SetString
cmd/compile: resolve TODO of Mpflt.SetString

Number literal strings returned by the lexer (internal/syntax package) and other
arguments to SetString never contain leading whitespace. There's no need (anymore)
to trim the argument.

Change-Id: Ib060d109f46f79a364a5c8aa33c4f625fe849264
Reviewed-on: https://go-review.googlesource.com/c/go/+/203997
Reviewed-by: Robert Griesemer <a6752097faebf1f02d80107d77a9be009089afe3@golang.org>

cloudflare/go,golang/go,akutz/go,CAFxX/go,nawawi/go,golang/go,golang/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,CAFxX/go,akutz/go,cloudflare/go,akutz/go,cloudflare/go,nawawi/go,CAFxX/go,akutz/go,akutz/go,nawawi/go,akutz/go,nawawi/go,golang/go,akutz/go,golang/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,cloudflare/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,CAFxX/go