Use '_' instead of '.' in symbol name sanitization

[termoshtt]

As commended in the rustc code:

Name sanitation. LLVM will happily accept identifiers with weird names, but gas doesn't!
gas accepts the following characters in symbols: a-z, A-Z, 0-9, ., _, $

rustc sanitizes symbol name for GAS output, but the rule of symbol names in PTX seems to be different from GAS especially around '.'. sanitize function in librustc_codegen_utils replace '-' and ':' into '.', but it causes problem as reported,
rust-lang#38789 (comment)
and this changes it to '_'.

[termoshtt]

uploaded

[Jasper-Bekkers]

Should we try to upstream this change?

[termoshtt]

• I am still checking this generates valid PTX
  • PTX validation test nvptx#12
  • the mangling issue seems to be resolved
• This change affects to all GAS output

I will report these after I success to generate valid PTX, but it may worth reporting separately.