Added LTO flags

[sej7278]

LTO functionality was added in IDE 1.6.10 to provide smaller sizes and faster code (and apparently build times). See issue #446

Will also work with ide 1.0.x as long as you have avr-gcc toolchain > 4.8 (e.g. debian/ubuntu)

On a nano328 for example:

Blink without LTO patch (HEAD):

Program:    1112 bytes (3.4% Full)
(.text + .data + .bootloader)

Data:         11 bytes (0.5% Full)
(.data + .bss + .noinit)

Blink with LTO patch:

Program:     946 bytes (2.9% Full)
(.text + .data + .bootloader)

Data:          9 bytes (0.4% Full)
(.data + .bss + .noinit)

This is only relevant to AVR.