add macro switch for low-memory devices other than __AVR__

[bimac]

Hi Paul,

not sure if this is useful, but I had some space issues compiling a project using SD.h on TeensyLC. By defining the macro SDFAT_LOWMEM the user can override the defaults and enable the limited feature-set on __arm__ platform as well.

Cheers,
Florian

[bimac]

Before:
Flash: [==========] 95.4% (used 60588 bytes from 63488 bytes)

After:
Flash: [======== ] 83.5% (used 53016 bytes from 63488 bytes)