runtime: rename vdso symbols to use camel case

runtime: rename vdso symbols to use camel case

This was originally C code using names with underscores, which were
retained when the code was rewritten into Go. Change the code to use
Go-like camel case names.

The names that come from the ELF ABI are left unchanged.

Change-Id: I181bc5dd81284c07bc67b7df4635f4734b41d646
Reviewed-on: https://go-review.googlesource.com/98520
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>