Fix open flags for rdi_from_dwarf

[tf2spi]

Hello all,

When experimenting with rdi_from_dwarf on Linux, I noticed that it failed to compile initially because clang handles __VA_ARGS__ differently on Linux than it does (for whatever reason...). After that, I tested rdi_from_dwarf with the following command line and got this as a response

$ build/rdi_from_dwarf --elf:build/rdi_from_dwarf --dump
error(input): could not load input file 'build/rdi_from_dwarf'

After an strace, I found the problem to be that os_file_open always used O_RDWR. This patch fixes lnx_flags to use O_RDONLY and O_WRONLY when appropriate.

[ryanfleury]

Fixed in 8f91ec4.