Below are some basic things how you can compile FPC, applying approximately to FPC 2.0 and Solaris 7 (SunOS 2.7). For more recent information see Lazarus on Solaris, which applies to 2.4 and Lazarus 0.9.30 (or later) on Solaris 10.
Alternatively, if you have Solaris 10 Sparc or later, you might choose to use the FPC 2.6.4 binaries contributed by a user, at http://www.bolthole.com/solaris/sparc-hints.html
Note that cross compiling is not easy, especially not since you need to link to libc.
Also the assembler and linker calls are not correct for assembling and linking on the target platform and may need some manual tweaking in ppas.sh and finally in t_sunos.pas.
Download the latest GNU binutils and build a cross-assembler and linker
Cross Compile the RTL
In the fpc/rtl/solaris/ compile the rtl with:
make BINUTILSPREFIX=i386-pc-solaris2.7- OPT=-g
Compile the famous hello world #1
Copy the solaris libc.* files to a dir on Linux
ppc386 -g -Xd -XPi386-pc-solaris2.7- -Fl<pathtosolarislibc> -Tsolaris helloworld
Run the executable on solaris (in gdb)
Compile the famous hello world #2
Another option is to compile on i386 and assemble and link on Solaris:
ppc386 -a -st -Tsolaris helloworld
This creates a ppas.sh script that can be run on Solaris.