Enable Link Time Optimization (LTO) compiler flag

[friedbunny]

Enables Link Time Optimization (LTO) for all targets, resulting in marginally smaller binary sizes and (theoretical) performance improvements.

• LTO increases the amount of time during the build process spent linking, but for a project this size it’s negligible — on my computer, it increased from 1.1 seconds to 1.9 seconds.
• ThinLTO did not show a measurable decrease in binary size, unfortunately, but “monolithic” did (and is what this PR enables).
• This change should provide improvements for consumer projects/libraries that build this Xcode project directly or consume it as a binary.
• There will be no change in the iOS Maps SDK — [build] Enable link-time optimization for iOS release builds mapbox-gl-native#12502 already did this (and, due how to this events library is consumed there, it is also LTO'd).

Binary size changes

The differences in the stripped binaries for the dynamic framework are not huge, but they’re something.

Before, LTO disabled

3.2M	MapboxMobileEvents
496K	MapboxMobileEvents-stripped
272K	MapboxMobileEvents-stripped-arm64
240K	MapboxMobileEvents-stripped-armv7

Monolithic LTO enabled

2.8M	MapboxMobileEvents
480K	MapboxMobileEvents-stripped
272K	MapboxMobileEvents-stripped-arm64
224K	MapboxMobileEvents-stripped-armv7

ThinLTO enabled

3.2M	MapboxMobileEvents
496K	MapboxMobileEvents-stripped
272K	MapboxMobileEvents-stripped-arm64
240K	MapboxMobileEvents-stripped-armv7

/cc @rclee @andrlee

[friedbunny]

Looks like this change decreases the static library’s raw binary size from 8.54 MB to 7.06 MB.

[rclee]

LGTM, Thanks Jason!
cc: @alfwatt @andrlee