CMake: append to LINK_FLAGS instead of replacing them

[Xiretza]

Description

With INTERPROCEDURAL_OPTIMIZATION/LTO enabled, LDFLAGS from the environment are ignored.

Related Issue

N/A

Motivation and Context

CMake takes the default linker flags from the LDFLAGS environment variable and initializes the targets' LINK_FLAGS with it. If set_target_property() is used to overwrite a target's LINK_FLAGS, these initial linker flags from the environment are lost.

How Has This Been Tested?

$ env LDFLAGS=-Wl,-z,relro cmake -B build -S . -DVTR_IPO_BUILD=on
$ grep relro build/vpr/CMakeFiles/vpr.dir/link.txt

Types of changes

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed