compiler: use SHA1-hash for symname for long gcbits symbols

compiler: use SHA1-hash for symname for long gcbits symbols

    The current scheme used by the compiler for "gcbits" symbols involves
    generating a symbol name based on a 32-char encoding of the bits data.
    This scheme works well in most cases but can generate very long symbol
    names in rare cases. To help avoid such long symbol names, switch to a
    different encoding scheme based on the SHA1 digest of the payload if
    the symbol size would be too large.

    Fixes golang/go#32083.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/177598

From-SVN: r271322